The present invention is generally directed to a mechanically-scanned directional antenna. More particularly described, the present invention is a scanning beam antenna using a rotatable combination of a lens and a reflective surface, such as a mirror, to scan a wide field-of-view.
Omnidirectional antennas cover a 360 degree field-of-view with a single beam. A directional antenna with a narrow azimuth beamwidth can be used to increase gain or provide directional information. For example, a 10 degree, half-power beamwidth antenna will have approximately 15 dB more gain than an xe2x80x9comnixe2x80x9d antenna with the same elevation beamwidth. FIGS. 1A and 1B emphasize this gain difference and illustrate the use of multiple narrow beams to maintain antenna coverage over a 180 degree field-of-view. As shown in FIGS. 1A and 1B, an antenna characterized by a narrow azimuth beam 14 of 10 degrees typically exhibits an increased gain when compared to a typical pattern 12 for an omnidirectional antenna. To obtain this desirable gain increase of a directional antenna over a wide field-of-view, multiple narrow azimuth beams 16 can be mechanically or electronically scanned to cover a 180 degree field-of-view.
Scanned antennas are typically implemented in one of two forms: electronic scan or mechanical scan. Electronically-gained antennas usually require beam forming networks which contain electronic RF switches or phase control devices. Mechanically-scanned antennas typically utilize a motor to rotate or position a directional antenna in different directions over the required field-of-view. Mechanically-scanned antennas are usually less expensive to construct than electronically-scanned antennas but have slower scanning ability and lower reliability due to the use of moving parts.
FIG. 2 illustrates an antenna with a typical reflecting mirror for scanning a narrow beam. Mirrored reflectors have been used to scan narrow beams over a small field-of-view. RF energy is normally collimated off the mirror, which can create spill-over loss if the mirror is tilted by a large angle. As shown in FIG. 2, a flared horn can emit an electromagnetic signal that is reflected by a reflecting surface, such as a mirror 22, to direct electromagnetic energy away from the antenna transmission axis. By rotating tie mirror 22 proximate to the output slot of the flared horn 20, the electromagnetic energy can be scanned over a relatively small field-of-view. Angle theta defines the angle between the mirror 22 and the reflection axis for the beam B reflected by the reflective surface of the mirror. To reflect electromagnetic energy from the reflective surface of the mirror 22 at an angle theta, the length L of the mirror 22 is defined by the ratio of the horn span H and the angle theta. The length of the mirror 22 is defined by Equation 1, as follows:                     H                  cos          ⁡                      (            theta            )                                              Equation        ⁢                  xe2x80x83                ⁢        1            
For example, if the angle theta is 45 degrees, then the length L is defined by 1.41xc3x97H. If the angle theta is 60 degrees, then the length L is defined by 2.0xc3x97H. If the angle theta is 75 degrees, then the length L is defined by 3.9xc3x97H.
Mechanical tracking antennas typically use motor-driven rotation with position knowledge or feed-back and can be commanded to point and dwell from one beam position to the next beam position. If the user application requires repetitive rapid scans of the same field-of-view, most mechanically-scanned antennas lose efficiency by decelerating at the end of the scan to allow a subsequent acceleration in the reverse direction. In addition to time inefficiency in reversing the angular momentum, the reverse rotation adds cost and complexity to the positioning system and causes wear and stress on the bearing and joints of the positioning system.
As shown in FIG. 3, a simpler and lower-cost tracking approach is provided by a continuous rotating, mechanically-scanned system. A horn antenna 32 can be rotated about a transmission axis by an RF rotary joint 34 driven by a motor 36. The rotation of the horn antenna 32 results in the sang of a narrow beam along a predetermined field-of-view. For example, to scan a 180 field-of-view with a 10 degree bean in the azimuth plane, the horn antenna 32 can be rotated in accordance with Option 1 by reversing the direction of the scan based upon deacceleration and acceleration operations completed by the RF rotary joint 34 and the motor 36. In the alternative, a horn 32 can be rotated to produce a continuous scan of the narrow beam in accordance with Option 2, thereby resulting in xe2x80x9cdeadxe2x80x9d time when the desired antenna coverage is 180 degrees. Although Option 2 has the advantage of re-scanning in the same direction, a recovery period results from the motion of the antenna when the desired antenna coverage is less than 360 degrees. The latency in revisiting a specific pointing direction is undesirable in a collision warning radar or missile detecting radar due to the closure movement of the target while a continuous rotating antenna is in the recovery period of its rotation. Recovery time can be reduced by faster antenna rotation but this increases cost, reduces reliability, and reduces the xe2x80x9cdwellxe2x80x9d time on the target due to the high angular rotation rate.
In view of the foregoing, there is a need in the art for an improved antenna that can efficiently scan an antenna beam over a wide field-of-view. Moreover, there is a need in the art to provide a mechanically-scanned antenna that can scan a narrow beam over a wide field-of-view in a reliable manner without the need for a complex positioning system. There is a further need in the art for a mechanically-scanned directional antenna that exhibits a near instantaneous reset or xe2x80x9cfly-backxe2x80x9d capability for applications requiring the re-scanning of a specific pointing direction. The present invention addresses these and other needs in the art by providing an antenna comprising at least one feed with a rotating dielectric lens having a reflective surface, such as a mirror, to scan a narrow beam over a relatively wide field-of-view.
The present invention addresses the needs of the prior art by achieving the desired characteristics of a low-cost, mechanically-scanned antenna with the high reliability and near instantaneous reset or xe2x80x9cfly-backxe2x80x9d capability of an electronically-scanned antenna. The present invention provides a low cost, reliable, mechanically-scanned directional antenna that can scan a wide field-of-view by rotating a reflecting lens/mirror assembly placed adjacent to a signal source. By keeping the signal source, such as a line source, stationary and scanning a lens/mirror assembly, the need for RF rotary joints or flexible transmission line and amplifier slip rings is eliminated for the antenna design. The lens can be implemented as one-half of a constant-K dielectric cylinder with a reflective surface or mirror, such as metal foil tape, applied to the flat portion of the lens. For example, a parallel-plate horn can scan 180 degrees of the azimuth plane by rotating a lens/mirror assembly positioned proximate to the horn output slot and within the transmission axis for the antenna beam. Installing a second half cylinder lens on the back side of this mirror can support the generation of two or more directional beams, thereby achieving a simultaneous scan of 360 degrees with the use of a pair of opposing horns. Switching the output of a single transceiver between two or more horns allows a xe2x80x9cfly-backxe2x80x9d re-scan capability.
In general, the present invention provides an antenna comprising a feed for delivering electromagnetic energy and a rotatable combination of a dielectric lens and a reflective surface. The combination of the dielectric lens and reflective surface, also described as a reflecting lens/mirror, is placed proximate to and in front of the energy feed. This supports the reflection of electromagnetic energy as the reflecting lens/mirror rotates over a predetermined range to scan the resulting beam within a desired field-of-view. For one aspect of the present invention, the energy feed is provided by a parallel-plate horn and the dielectric lens has a half-cylinder shape. The reflective surface is typically placed adjacent to the flat surface of the half-cylindrical lens to form the reflecting lens/mirror assembly. The cylindrical portion of the lens can face the energy feed, thereby separating the reflective surface positioned along the flat surface of the lens from the energy source by slightly more than the radius of the lens. The reflecting lens/mirror assembly can be rotated about the energy source by a mechanical rotating mechanism, such as a motor coupled to a belt-drive. By rotating the reflecting lens/mirror assembly over a range of 90 degrees, the parallel-plate horn can scan a narrow beam over a range of approximately 180 degrees. The antenna scan rate and angular movement is twice the rotation rate/movement of the lens/mirror.
For another aspect of the present invention, a second half-cylinder lens can be placed adjacent to the rear of the reflective surface to form a cylindrical lens comprising a reflective surface positioned between a pair of half-cylinder lens. Two or more energy feeds, such as horn antennas, can be positioned proximate to this, rotatable combination of cylindrical lens and a reflective surface to provide two or more directional beams that scan a wide field-of-view in response to rotation of the cylindrical lens/reflective surface assembly. A switch can be used to switch a signal source between two or more energy feeds to enable a xe2x80x9cfly-backxe2x80x9d re-scanning operation.
That the invention provides an antenna comprising an energy feed and the rotatable combination of a lens and a reflective surface will become apparent from the following detailed description of the exemplary embodiments and the appended drawings.